I need to remain in Canada for 3 years to be eligible to apply for citizenship. Once I have dual US/Canadian citizenship, I can stay in whichever country I like for as long or as little as I please.
Just finished reading "The Border Guide" by Robert Keats, which goes into detail (almost excruciatingly so) about the advantages/disadvantages of living on each side of the border. All you have to consider is estate planning, currency risk, retirement plans, social programs, health care, cost of living, tax rates, and the weather. Easy as pie.
Right now we're liking Canada better, even though it costs maybe 5% more to live here. Taxes, food and utilities are all higher than the States, but medical insurance is much lower.
Once we're 65 and eligible for Medicare, we may spend 7-8 months in Canada and 4-5 months in the sunny Southwest US since we won't have to buy travel insurance.